The invention relates generally to the field of ignition spark timing circuits for internal combustion engines, and more particularly to the field of electronic ignition spark timing circuits in which a predetermined advance angle versus engine speed characteristic is produced. U.S. Pat. Nos. 4,104,997 and 4,125,097 illustrate various embodiments of such circuits.
In general prior art electronic spark timing circuits such as those referenced above have provided a tremendous advantage over the previous spark timing circuits in which the advance angle versus engine speed variation was mechanically created by using a centrifical force created by the engine crankshaft rotation to physically displace a cam which controlled the amount of engine spark advance. In the electronic spark timing ignition circuits illustrated in the two previously referenced U.S. patents, generally a varying signal, an analog signal in both patents, serves as an input to a switching device or comparator which either has an internal switching threshold (such as the base emitter junction voltage of a transistor) or an external switching threshold (such as an external voltage applied to one of two terminals of a comparator). These prior art circuits have either provided no hysteresis for the switching devices, such as the ignition circuit shown in U.S. Pat. No. 4,125,097, or they have provided a constant hysteresis effect for the switching device, such as the comparator utilized in U.S. Pat. No. 4,104,997.
In general, providing a hysteresis effect for the switching comparator has been desired since this feature minimizes noise falsing of the switching comparator. In other words, once the comparator has determined that it should change its state, the comparator will remain in the changed state until a different and substantially larger switching threshold is reached. It has been found that while hysteresis is a desirable quality for the comparator utilized in U.S. Pat. No. 4,104,997, this hysteresis characteristic of the comparator will result in limiting the sensitivity of the comparator. This occurs because due to the hysteresis effect, which prevents small magnitude noise pulses from disrupting the operation of the comparator, the comparator will also ignore small amplitude signals which may be generated as the proper inputs to the comparator during extremely high engine speeds. Thus in order to have the prior art spark timing circuits function properly at high engine speeds, the noise immunity of these circuits had to be compromised.